1. Field of the Invention
The present invention pertains generally to industrial roll covers and, more particularly, to fillers provided in elastomeric roll covers used on paper machine rolls.
2. Description of the Related Art
Industrial rolls are used in many manufacturing processes, including steel mills, textile mills, and others. Paper machines in paper mills use a wide variety of different types of rolls in different environments for different purposes. Rolls on paper machine can be used for merely transporting the web from one location to another, or can be used in direct contact with the web or with paper machine clothing to treat the web, such as by pressing. The conditions under which paper machine rolls function can be quite harsh, including high temperatures and high pressures in the presence of corrosive chemicals. In addition, operating speeds are high, and ever increasing, and paper machine operation is substantially continuous.
It has been known to use steel roll shells or cores covered with different materials suitable for the particular location, operating conditions and roll performance requirements of the paper machine roll. Using a cover on the roll allows for quicker and less expensive reconditioning of the roll than replacing the entire roll. When a roll cover is damaged or worn, it can be reground easily, or stripped from the shell and replaced at lower cost than replacing the entire roll. By using different roll cover materials and formulations, the surface characteristics of the roll can be optimized for the location in the paper machine in which the roll is installed. Thus, desired and necessary hardness, abrasion and wear resistance, chemical resistance and other properties can be achieved. Both natural rubbers and synthetic elastomers have been used in paper machine roll covers. It also is known to use a plurality of different materials in layers between the roll shell and the top layer of the roll cover, as transition layers between the shell and the top layer, to promote roll cover life.
Efforts continue to make paper machines run faster, and to run for longer intervals between scheduled maintenance procedures. Chemicals used in the paper making process as conditioners, treatments and additives also are changing and improving. Thus, the demands on the performance of rolls and the covers on the rolls are also ever increasing. Synthetic fiber or particle fillers have been used mixed with the elastomer to improve paper machine roll performance, and to increase roll cover life.
It also is known to provide roll covers having a top layer made of a mixture of elastomeric material and ultra high molecular weight polyethylene (UHMWPE). According to one known composition range, the mixture includes about 100 parts elastomeric material by weight, and between about 25 and 50 parts UHMWPE by weight. Such roll covers may have a hardness between 10 to 50 P&J hardness scale.
While a roll cover comprising a mixture of an elastomer and UHMWPE as disclosed above has utility in paper machines for some paper making processes, and in some positions on the paper machine, it has certain shortcomings as well. A paper machine roll cover directly contacting the paper web must exhibit good sheet release properties. That is, the paper web, which may still contain a substantial amount of water and have minimal sheet strength, must release from the roll surface. If the web sticks or adheres to the roll surface, a web brake will occur, with the resultant waste of manufacturing time and product. Due to the large particle size of UHMWPE commonly used in rubber compounds, exposed particles on the cover surface limit the attainable surface smoothness, and thereby the sheet release properties of the roll cover. Adhesion between the elastomer matrix and the UHMWPE particles can be inconsistent, resulting in surface marking on the cover that translates to defects in the web being processed. Additionally, UHMWPE fibers and particles are expensive to obtain, thus substantially increasing the cost of the roll cover. Since paper machine roll covers commonly are re-ground with some frequency, and may require re-covering after a limited number of re-grindings, the additional cost from the use of UHMWPE fibers or particles can increase substantially the cost of operating a paper machine.
It also is known to provide roll covers comprising a polymeric material into which have been dispersed polyethylene particles that have been surface treated to alter the properties of the mixture. For example, U.S. Pat. No. 4,880,879, entitled “ABRASION RESISTANT COMPOSITE MATERIAL AND PROCESS FOR MAKING THE SAME”, issued Nov. 14, 1989, discloses a material and a process for the material, having polyethylene particles in a thermoset polymeric matrix, the particles having been surface treated to enhance the bonding between the particles and the polymeric matrix. While the particles are called “high molecular weight polyethylene (HMW PE) particles”, the molecular weight of the particles is said to be at least 1 million, and preferably in a range of 1 to 9 million. Thus, a roll cover made from such a mixture would suffer from the same disadvantages as the elastomeric-UHMWPE roll covers discussed above.
What is needed in the art is a paper machine roll cover that is economical to fabricate, is reliable when operated under adverse conditions, resists marking and damage, and can be ground to a high degree of smoothness.